A copier, fax machine, printer, etc., are now essential equipment for paperwork. These days, an image forming apparatus working as a complex machine having all the functions provided by those copier, fax machine, printer, etc., has been widely noticed. This image forming apparatus has a function of recording an image of characters and figures on a sheet of recording paper, and is capable of recording a monochrome image and a color image. In order to deal with increasing paperwork efficiently, it is required to improve the processing speed of an image recording apparatus. It is especially required to improve the processing speed for color printing and therefor, the image formation process using a tandem method has been widely employed.
An image forming apparatus using the tandem method simultaneously forms toner images of four colors of yellow (Y), magenta (M), cyan (C), and black (K) on dedicated photosensitive drums, and superimposes the toner images on an intermediate transfer object (e.g., endless belt) or directly on a recording paper. By employing this method, it can be expected to achieve printing speed four times faster than those of other methods.
The image forming apparatus using the tandem method forms an image using electrostatic power. Because of this, a characteristic changes due to a service status of each processing unit or environment and that readily leads to a change in the concentration of a formed image. To prevent such trouble, for example, process control in image formation is carried out by forming a patch image (square of several mm in length and breadth) for toner concentration detection when the image forming apparatus is started or after it is operated for a given time, by detecting the toner concentration of the patch image with an optical sensor, and by adjusting control conditions for components of an image forming unit on the basis of the detected toner concentration (e.g., see Japanese Laid-Open Patent Publication No. 2003-149952).
When toner images colored in Y, M, C, and K are formed on the photosensitive drums, respectively, and are transferred to the transfer object or a recording paper to superimpose pixels, a color irregularity and/or color shift occurs on a color image if the superimposition of pixels (square of about 42 μm in length and breadth) is not carried out properly. This makes impossible the reproduction of a desired color. A shift in pixels is often caused by a temperature increase in the image forming apparatus, and the amount of the shift varies according to the type of equipment. Such a shift, therefore, must be corrected to same extent. Thus, a position shift in pixel superimposition is corrected by forming respective patterns colored in Y, M, C, and K on the transfer object, and reading the patterns with a transmittable or reflective optical sensor to detect the amount of shift of each color (e.g., see Japanese Laid-Open Patent Publication No. H10-333391).
In the image forming apparatus using the tandem method for image formation, each of the photosensitive drums for Y, M, C, and K is driven by a separate pulse motor (also called step motor). In this case, the pulse motor driving the photosensitive drum for K may be controlled independently while the other pulse motors driving the photosensitive drums for Y, M, and C are controlled in common. After a toner image formed on each photosensitive drum have been transferred to an intermediate transfer belt or directly to a recording paper, the photosensitive drum is cleaned in preparation for the formation of the next toner image. Upon cleaning the photosensitive drum, residual toner on the surface of the photosensitive drum is scraped off. This process applies heavy load to the pulse motor driving the photosensitive drum.
A pulse motor is an electric motor that operates in synchronization with an input pulse train, and is capable of exact positioning control in a simple circuit configuration, thus often used as a positioning means for a rotor. This pulse motor, however, loses control by getting out of synchronization, and occurs step out such as a halt when load on the motor is too heavy or the motor becomes incapable of catching up with incoming pulse train in synchronized move. The load on the pulse motor is likely to increase, particularly, when the photosensitive drum for K consuming a greater amount of toner is cleaned. This case leads to printing of a defective image due to a position shift of an image, which requires the suspension of printing by quickly detecting an abnormality. To quickly detect a rotation failure (abnormal rotation) of the photosensitive drum, providing the pulse motor with a detecting means for detecting fault is effective. This approach, however, poses a problem of hampering the miniaturization of the image forming apparatus and increasing costs.
When an abnormal rotation of the photosensitive drum occurs during printing of such image data received through a communication line as fax data, a toner image to be formed on the transfer object becomes disordered one, which may result in the loss of the image data. An image received through fax transmission is usually deleted from a memory device once printing is over even if a fine image cannot be obtained because of a rotation failure of the photosensitive drum due to out-of-step operation of the pulse motor. This makes it impossible for a user to retry the image forming process all over again like reprinting or recopying image data, etc., at hand. This causes a problem that is difficult for the user to reacquire the image data except by asking a transmitting party to retransmits a fax image.